Description:
For each of the past 3 months (the last month is shown on plot) monthly precipitation is first ranked by percentiles.  (If a given month is in the 0th percentile its the driest in the base period while the 100th percentile indicates the wettest.)  Next, for each gridpoint, a count is made of the number of times in the past 3 months that the precipitation was in the bottom or top tercile.  In the count, months with precipitation in the middle tercile are ignored. 

On the plot, light and dark brown shading represents that 2 of the past 3, and 3 of the past 3 months were in the bottom tercile, respectively.  Light and dark green shading on the plot indicates areas where precipitation was in the top tercile for 2 of the past 3, and 3 of the past 3 months, respectively. Regions with grey shading typically receive less than 3.0 cm of precipitation over the course of the previous 3 months. 

The precipitation anomalies are from the CAMS-OPI dataset.  The period used for computing the terciles is 1961-1990.  "CAMS" is an acronym for the "Climate Anomaly Monitoring System" in use at the Climate Prediction Center (CPC) at the US National Centers for Environmental Prediction (NCEP).  "OPI" stands for "Outgoing longwave radiation Precipitation Index" (the statellite precipitation estimates are based on emitted longwave radiation observed by polar-orbiting satellite.)